Wireless power supply devices can be realized by means of inductive and/or capacitive proximity coupling. This is used in many RFID systems and wireless battery chargers. In this case, a source unit generates an alternating electromagnetic field. This alternating electromagnetic field is coupled through coupled coils (inductive coupling) or by an open capacitor (capacitive coupling) to a load, in the following referred to as a load unit. With increasing distance from the source unit to the load unit decreases the coupling strength and reduces the receivable amount of power at the load unit. In the case of an open capacitor thereby minimizing the coupling capacity, and in the case of coupled coils thereby increasing the leakage inductance. It is known that this effect can be compensated in case of the leakage inductance by a compensation capacitor and in the case of the coupling capacitor by a compensation inductor. This results in at least one resonant circuit at the source unit side and at least one resonant circuit at the load unit side of the power transmission system. These resonant circuits compensate the leakage inductance and coupling capacitance under the condition that the resonant circuits are tuned exactly to the same resonance frequency and the source unit of the power transmission system operates at this resonance frequency. Such coupled resonant circuits are the base of band filters and have been used for many years for e.g. the coupling of amplifier stages etc.
In “Wireless Power Transfer System Description” of the “Wireless Power Consortium (WPC)”, a resonant circuit is shown, which is driven by a generator. Here, a plurality of part inductors is selectively used in a resonant circuit in order to concentrate the radiated energy field to the surface where load units are placed. Further, the power supply is controlled by an upstream voltage or current regulator.
The general disadvantage of wireless power principles, based on coupled resonant circuits in the source- and/or load unit, is the resonance frequency detuning due to component tolerances, component aging, coupling and load changes. This detuning effect is undesirable because the impedance of the resonant circuit is frequency selective and a predetermined operation frequency does no longer coincide with the resonant frequency of the circuit. Consequently, the power transmission system operates no longer toward a real load resistance, but also toward an inductive or capacitive component. Thus, the resulting reactive power increases the power dissipation. This lowers the overall efficiency in the source unit (resonant circuit driver stage, etc.) and thus reduces the efficiency of the entire power transmission link. In addition, distortions increase, because the driver circuits generate more harmonics.
The known network sensing method measures the resonance frequency of the network during a time interval and operates the system at this resonance frequency thereafter, but the system has no ability to control the resonance frequency of the resonant circuit or network actively. This would be very desirable, due to guidelines such as EN300330, REC7003 and ITU-RSM2123, which determines maximum power levels over frequency ranges (e.g. 119 . . . 135 kHz).
Furthermore, there exist national specific constraints for narrow frequency ranges within a frequency band that require much lower power level limits. It is therefore important to control not only the power level but also the spectral position of the emitted power.
Another problem relates to the product of the variable coupling (k) and the quality (Q) in the further description referred to as energy coupling (k·Q). The quality factor Q is a measure of the energy stored in the system and the energy transferred by the system, or in other words a measure of the reactive power circulating in the resonant circuit and output power of the resonant circuit. The coupling (k) is essentially determined by the geometry of the wireless coupling link such as distance (area, distance), its angular orientation and the coupling medium. A change in the load resistor in a resonant circuit will also change the energy coupling (k·Q). The system is more or less damped and the energy coupling (k·Q) is consequently smaller or larger. A desired constant output voltage or a desired constant output current of the load unit requires therefore the control of the output power in the source unit by means of a data-load modulation link and/or the control of the output voltage and/or current on the load unit side.
In “A Frequency Control Method for Regulating Wireless Power to Implantable Devices” proposes frequency detuning in the source unit for the output voltage regulation. The problem with this solution is the detuning of the resonance frequency that also simultaneously controls the power of other coupled load units if more than one load unit is used in the wireless power transmission link. An independent control is not possible. Thanks to the performed detuning on the load side, several load units are independently adjustable. However, disadvantageously, the source unit is no longer loaded with a real load resistance in the wireless power transmission link in both variants. The energy transfer is no longer based on coupled resonant circuits and the overall network does not operate in the real domain (resonance case). Consequently, this causes higher losses in the source unit and/or in the load unit.
In “Wireless Power Supply for Implantable Biomedical Device Based on Primary Input Voltage Regulation”, the output voltage of the load unit is digitized and transmitted to the source unit. The operating voltage of the source unit is controlled based on the received data of the load unit. This approach is pursued in the standard of “Wireless Power Consortium (WPC)”, wherein a wireless power transmission link up to 5 watts is specified. This approach is efficient because only the amount of power is transmitted as needed. Unfortunately resonance detuning is not considered.
Another fundamental problem is not considered in all known systems, and relates to the upper energy coupling boundary value (k·Q=1), wherein the maximum possible power can be transmitted without any substantial frequency detuning or without substantially bandwidth increase. This case is very important because a small coupling factor (k) can be compensated with a higher quality (Q). In this manner, the energy coupling (k·Q) can be held constant. Thus, for example, the power transmission distance can be increased.
In practice, however, often results in a dynamic coupling (time-varying coupling (k)) and/or dynamic load. Examples are different wireless power transmission links with dynamic coupling due to variable geometries, and/or variable distance between the source unit and load unit, and/or modifying the coupling media and especially by altering the load resistance.
It is an essential desire to design a wireless power transmission system as universal as possible and ideally approximating a wired connection as much as possible. This would automatically result in the best efficiency and additionally serve maximal flexibility.
It is further desirable that for a given source unit the operational distance of the power transmission link can be optimized in designing the load unit without any design modifications in the source unit (coils and/or capacitors). E.g., a load unit featured with a small coupling (k) (small coupling surface, etc.) can compensate its limited range with a greater quality factor (Q). This would meet requirements of an open flexible standard, wherein only the basics shall be specified. Additionally, multiple arbitrary load units coupled under changing coupling conditions (changing coupling factors (k) and/or changing qualities (Q)) shall maintain operable in parallel on a source unit.
Further, it should be possible to specify the transmission frequency, as there are radiation limits by law (amplitude and frequency), such as EN300330, REC7003 and ITU-RSM2123.
Further it should be possible to vary the transmission frequency in order to reduce the spectral peak power.
The following invention describes methods and their implementation details that meet the mentioned requirements. The methods described in the following invention and its detail implementations are featured by low-cost implementations, stable operation and high efficiency.